The present invention relates to traveling wave tubes and more particularly to circuits for such tubes to improve their performance. It is known that undesired backward waves, as well as the desired forward waves, are present on the slow wave structure (SWS) of a traveling wave tube (TWT). If backward waves are permitted to propagate all the way from the TWT output end to the input end undesired oscillations may result. In standard TWT design practice the SWS is interrupted or severed into two or more sections. The length of each section is limited so that feedback due to the backward wave, in any section, is not large enough to lead to oscillation. In present TWT design practice (the prior art) forward and backward wave power incident on each sever is dissipated in lossy resistive terminations. The resultant loss of backward wave power has desirable effects on TWT performance. However loss of forward wave power has a rather complicated adverse effect on TWT efficiency. In some TWT designs the above adverse effect may be substantial. A theoretical discussion of this adverse effect is given in a publication by Scott: ("Why a Circuit Sever Effects Traveling Wave Tube Efficiency" by A. W. Scott. IRE Transactions ED-9, Jan. 1962, pp. 35-40.)
Scott determined that the above adverse effect can be minimized if a certain minimum length is provided for the final SWS section between the final sever and the tube output. However the above minimum length requirement may conflict with the maximum length requirement imposed by the need to prevent oscillations. Partly because of the above dilemma TWT researchers have long sought to develop circuit severs for TWT's which would dissipate only the power in the backward wave while permitting the forward wave to pass through the circuit sever with only modest attenuation. However the developmental efforts to date have all involved incorporation of ferrite material within a TWT and none have been successful enough to produce any apparent effect on TWT design practice. The abovementioned development efforts are described in the following reference and in references given therein: "Distributed Ferrite Isolation in Traveling Wave Tubes" Contract N00014-74-C0322. Annual Status Report. Sept. 2, 1975.
It is also known that TWT's tend to lose, as the beam progresses along the SWS, the desired synchronization between the electrons of the beam and the interacting wave. This tendency imposes a limit to the efficiency obtainable from a given TWT design. Various remedies have been proposed to enhance TWT efficiency by restoring, or delaying the loss of, synchronization between the beam electrons and the SWS wave. The remedies include abrupt increase in beam electron velocity, and various methods to provide abrupt or gradual decrease in SWS wave velocity. Discussions of these remedies may be found in the following publications: (1) "Improvement of Traveling Wave Tube Efficiency Through Period Tapering" N. H. Pond and R. J. Twiggs. IEEE Trans,. Electron Dev. Vol. ED-13, 1966, pp. 956-961. (2) "Applied Research on Efficiency Improvement in O-Type Traveling-Wave-Tubes", Hughes Aircraft Co. (Report RADC-TR-67-259) Apr. 1967. See also U.S. Pat. No. 3,846,664 issued Nov. 5, 1974 to King et al.